This invention relates to a control device for a vehicle mounted a.c. generator, and particularly to such a control device capable of regulating the field current and drive torque characteristics of the generator in an initial stage of its operation such that these characteristics are made to approximate those obtainable when the generator operation is steady.
FIG. 1 shows a circuit diagram of a conventional control device of this type, which comprises an a.c. generator 1 driven by the vehicle engine, a fullwave rectifier 2 for rectifying the output voltage of the generator, and a voltage regulator 3 for regulating the rectified output voltage to a predetermined value. The generator 1 includes an armature winding 101 and a field winding 102. The rectifier 2 has a main output terminal 201, an output terminal 202 connected to the field winding 102 and to the voltage regulator 3, and a third output terminal 203 which is grounded. The regulator 3 includes series connected resistors 301 and 302 which constitute a voltage divider for dividing the voltage at the output terminal 202 of the rectifier 2, a Zener diode 303 for detecting the junction voltage of the divider, which is a fraction of the output voltage at terminal 202 of the rectifier 2, so as to be turned on when the detected voltage exceeds a predetermined value, a transistor 304 which is turned on when the Zener diode 303 turns on, an output transistor 305 which is controlled by the transistor 304 to on-off control the current supply to the field winding 102 of the generator 1, a resistor 306 connected to a base of the output transistor 305, and a diode 307 connected in parallel to the field winding 102 for absorbing on-off surges produced in the field winding 102.
In the same figure, numerals 4, 5, 6, 7 and 8 depict a vehicle mounted battery, various electrical loads of the vehicle, a key switch of the vehicle, a resistor for energizing the field winding 102 of the generator at the start time of the generator operation, and a reverse current blocking diode, respectively.
The output current and drive torque characteristics of the generator in FIG. 1 when it is operating with a maximum load condition are shown in FIG. 2, in which the dotted curves show the current and torque when the generator is cold, i.e., in an initial stage of operation, and solid curves show them when the generator is hot, i.e., warmed up and operating steadily.
In operation, when the key switch 6 is closed to start the engine (not shown), an initial field current flows from the battery 4 through the switch 6, the resistor 7 and the diode 8 to the field winding 102 of the generator 1, and the latter is enabled to generate power. Then, when the engine starts to rotate, the generator 1 is rotated to thereby generate an output voltage which is rectified by the rectifier 2 and supplied from the output terminal 202 to the voltage regulator 3 and the divider 301, 302. When the fraction of the rectified voltage from the divider exceeds the predetermined value which is determined by the resistors 301 and 302 and the Zener diode 303, the transistor 304 is turned on.
On the other hand, when the fraction is below the predetermined value, the Zener diode 303 and thus the transistor 304 remain turned off.
The output transistor 305 is on-off controlled by the transistor 304 to thereby on-off control the field current flowing through the field winding 102 of the generator such that the output voltage of the generator is regulated to a predetermined value.
The voltage regulator 3 performs the above mentioned operations regardless of the condition of the generator, so that the generator 1 supplies the regulated output voltage through the output terminal 201 of the rectifier 2 to the battery 4 and the various electrical loads of the vehicle, which are depicted by reference numeral 5.
As will be clear from FIG. 2, in the cold state of the generator which is immediately after startup, the output current of the generator gradually increases with rotational speed as shown by the dotted line and, when it is heated by itself and by an increasing ambient temperature and becomes hot, the curve is gradually reduced to that shown by the solid line. The nominal power of the generator is generally determined according to the characteristics thereof in a hot state. That is, the differences between the output characteristic curves of the generator in a cold state and in a hot state represent a mere margin for assuring the hot state characteristics and, therefore, such margin should ideally be removed.
As to the drive torque which is determined by the magnetomotive force generated by the field current, and which increases rapidly and then decreases with the rotational speed of the generator, it is maximum in the cold state and gradually decreased to the solid curve with increase in temperature. The drive torque of the generator corresponds to the additional mechanical torque to be generated by the vehicle engine. That is, there is a difference in drive torque between the cold state and the hot state, which affects the engine as an extra load. Particularly, since immediately after the engine starts, the torque of the engine is relatively stable and the drive torque of the engine is generally high in the cold state, the latter torque influences the engine operation such that the smoothness of the engine rotation is degraded and fuel consumption is increased. These problems are usually amplified when the engine is very cold.
It might be possible to on-off control the field current by detecting the latter and cutting off the output transistor of the voltage regulator directly when the detected value is higher than a predetermined value so that the field current is cut off when such a situation occurs, as disclosed in U.S. Pat. No. 4,570,199 issued to Morishita et al, two of whom are co-inventors of this application. However, such a system is only suitable when the field current is suddenly and abnormally increased, such as by a short-circuit of the field winding.